1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for transmitting Reference Signals (RSs) for data demodulation and a method and apparatus for receiving RSs for data demodulation.
2. Discussion of the Related Art
Multiple Input Multiple Output (MIMO) has recently attracted interest to maximize the performance and communication capacity of a wireless communication system. Compared to conventional use of a single Transmission (Tx) antenna and a single Reception (Rx) antenna, MIMO adopts a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. A MIMO system is called a multiple antenna system. MIMO is an application of putting data segments received from a plurality of antennas into a whole message, without depending on a single antenna path to receive the whole message. Consequently, MIMO can increase data transmission rate within a given area or extend system coverage at a given data transmission rate.
MIMO schemes are classified into transmit diversity, spatial multiplexing, and beamforming. Transmit diversity increases transmission reliability by transmitting the same data through multiple Tx antennas. In spatial multiplexing, multiple Tx antennas simultaneously transmit different data and thus high-speed data can be transmitted without increasing a system bandwidth. Beamforming is used to increase the Signal-to-Interference plus Noise Ratio (SINR) of a signal by weighting multiple antennas according to channel states. Weights may be expressed as a weight vector or a weight matrix, called a precoding vector or a precoding matrix.
Spatial multiplexing is further divided into spatial multiplexing for a single user (or Single User MIMO (SU-MIMO)) and spatial multiplexing for multiple users (or Multi-User MIMO (MU-MIMO)).
A Base Station (BS) may transmit a plurality of layers for one or more users. For this purpose, the BS multiplexes the layers into a predetermined time/frequency area and transmits the multiplexed layers to one or more User Equipments (UEs). In general, maximum transmission power available for downlink transmission of the BS is determined by the supported frequency bandwidth, data throughput, and power efficiency of the BS. Because the total transmission power available to the BS is limited to a predetermined value, the BS needs to efficiently allocate transmission power to each subcarrier in an Orthogonal Frequency Division Multiplexing (OFDM) symbol interval.
To demodulate data allocated to a predetermined time/frequency area, a UE estimates the configuration of physical antennas used for the data transmission and channel quality using an RS received from the BS, that is, the UE performs channel estimation using the received RS. Channel estimation and an RS will be described in brief. To detect a synchronization signal, a receiver should have information about a radio channel (e.g. the attenuation, phase shift, time delay, etc. of the radio channel). Channel estimation is the process of estimating the magnitude and reference phase of a carrier. A wireless channel environment is characterized by irregular variations of channel state over time, called fading. The amplitude and phase of the fading channel are estimated through channel estimation. That is, channel estimation refers to estimating the frequency response of a radio interface or radio channel. For channel estimation, a reference value is estimated using some RSs of a BS by a two-dimensional channel estimator. An RS is defined as a symbol with high power without carrying actual data in order to help carrier phase synchronization and BS information acquisition. A transmitter and a receiver can perform channel estimation using such RSs. The receiver can recover data received from the transmitter based on the result of RS-based channel estimation.